Post image for R-Value is a Lie (And Other Provocative Titles)


Or at least as Allison Bailes posted earlier this year, it is not a constant.

Green Building Advisor once published my post on thermal bridging. The audience there is an extraordinarily knowledgeable one which leads to some spirited discussions. Following the thermal bridging article, there was some jockeying over whether R-value is an objective value and if it is over-rated. My belief is that R-value measuring a material’s thermal resistance is just one material property that needs to be considered.

There’s air permeance, vapor permeance, water resistance, etc. Some commentators opined that being overly concerned about some of these other properties colors the fact that fiberglass is largely effective. Or is it?!? (*DUN DUN DUNNN!!*)

Building Science Corp to the Rescue

It is. Building Science Corp, notorious through the building industry for pursuing building science, released some findings of their insulation material testing. What they’re doing is that having constructed one of the most advanced testing facilities in the world, Building Science is putting building materials to the Building Science Test (TM). One verdict: temperature matters.

Temperature matters for insulation performance but not uniformly for all types of insulation. First, the good news. Fiberglass, cellulose, Expanded polystyrene (EPS) and Extruded Polystyrene (XPS) foam board performed better than the R-value that was tested at lower temperatures. The bad news? Polyisocyanurate, an insulation with one of the highest R-value per inch and very commonly used for exterior sheathing, performed below the tested R-value. Not so good.

Polyisocyanurate Foiled

What does this revelation mean for insulation and green building? Polyisocyanurate is primarily used for exterior sheathing insulation. In warm climates, this shouldn’t be a problem. Cooling climates that are more concerned with the hot summers than cold winters won’t have any issues. Knock yourself out with the exterior insulation and when the January weather dips down to 45, well, that is tough.

In cold climates, this changes. Polyisocyanurate insulation performance drops as the temperature drops meaning 6 inches of exterior foam board may not be delivering the R-value advertised on the label.What does this mean for construction and design? The main upshot is that if you use polyisocyanurate insulation, plan for reduced performance in cold climates. Design for an extra layer of polyiso or some similar adjustment to hit the target R-values.

It would be nice if there were one number with which to gauge the effectiveness of insulation. It has many different properties like any material. That the r-value flucuates with temperature may be a bit disconcerting but ultimately requires only a modest adjustment in your planning for home projects.